1. Field of the Invention
The present invention relates to a honeycomb structure in which a plurality of honeycomb segments are integrally bonded to one another by bonding material layers, more particularly to a honeycomb structure which is useful as a filter for trapping an exhaust gas, above all a diesel particulate filter (DPF) for trapping particulate matters (particulates) and the like in an exhaust gas from a diesel engine and in which generation of defects such as cracks due to a thermal stress during use and during regeneration is effectively prevented.
2. Description of the Related Art
A honeycomb structure is incorporated into an exhaust system or the like of a diesel engine, and used as a filter for trapping an exhaust gas, for example, a diesel particulate filter (DPF) in order to trap and remove particulate matters (particulates) included in an exhaust gas from a diesel engine or the like. Such honeycomb structure has a disadvantage that temperature easily non-uniformly rises in the whole honeycomb structure, and defects such as cracks are generated by a thermal stress during use (when the particulates are trapped and removed) and during regeneration (when the particulates deposited in the filter are burnt and removed in order to remove an increase of a pressure loss due to the particulates deposited in the filter with an elapse of time). From a viewpoint of solving such disadvantage, there is proposed a honeycomb structure in which bond surfaces of a plurality of honeycomb segments are integrally bonded to one another via bonding material layers to constitute a bonded honeycomb segment article, and accordingly the thermal stress is reduced.
However, a honeycomb structure made of silicon carbide (SiC), which is expected to be used owing to an advantage that its thermal resistance is excellent, has disadvantages that a thermal expansion coefficient is larger than that of a honeycomb structure made of cordierite and that a resistance to thermal shock is inadequate. Therefore, it is disadvantageously difficult to sufficiently use the advantage. In recent years, with enlargement of the filter, the thermal stresses generated during the use and during the regeneration have increased more remarkably than before. Consequently, a frequency and a degree of generation of the defect due to the thermal stress rapidly raise a serious problem. To solve such disadvantage, there is disclosed a method of manufacturing a ceramic structure (honeycomb structure), including a step of assembling ceramic blocks by repeating steps of: forming an adhesive paste layer (bonding material layer) on a side face of a porous ceramic member (honeycomb segment) so that an area of the layer occupies 60% or more of a total area of this side face; and thereafter laminating another porous ceramic member on the layer (see Patent Document 1). However, the ceramic structure (honeycomb structure) disclosed in Patent Document 1 has a problem that the generation of the defect due to the thermal stress cannot be necessarily sufficiently prevented for a reason that a temperature gradient in the filter is easily increased. This is because a rigid bonded structure has an opposite effect of increasing the thermal stress generated from the temperature gradient, and the bonding member has a large thermal capacity and a low temperature drop/rise rate as compared with a porous ceramic member.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-219317